The invention relates to a new semiconductor material based on amorphous germanium, preferably for red-sensitive photovoltaic components, and a method for its production.
In order to reduce the production costs for solar cells, and hence the costs of providing photovoltaically generated energy, suitable materials are being sought for thin-film solar cells based on amorphous semiconductors. These materials are intended to replace solar cells consisting of crystalline semiconductor material which, although powerful, are also expensive and are not competitive in comparison with other energy sources. They have numerous advantages in production and are distinguished, for example, by low process temperatures of less than 250.degree. C. and low material consumption. Layer thicknesses of less than 1 .mu.m are sufficient for complete absorption of light. In addition, it is possible to produce large-area semiconductor films on cheap substrates, for example window glass, by suitable deposition methods.
A disadvantage of amorphous solar cells is their relatively low efficiency compared with crystalline solar cells. With small areas, the efficiency can reach 12 percent, but the efficiency is only 9 percent in the case of a 1000 cm.sup.2 module. A further disadvantage is the lack of light stability of the cells. Even the best of the known solar cells of amorphous semiconductor material exhibit 15 to 20 percent ageing within the first few months when used in the field, which becomes noticeable by a reduced efficiency.
A possible cell structure which avoids these disadvantages is the tandem cell. In this case, a plurality of solar cells each with a different energy gap, and hence different spectral sensitivity, are connected one behind the other. A combination of amorphous silicon with an energy gap of approximately 1.75 eV and a cell of an amorphous silicon-germanium alloy or of amorphous germanium itself with an energy gap of between 1.0 and 1.2 eV, connected downstream from it, would, for example, be suitable. Using this tandem cell structure, theoretical efficiencies of over 20 percent are possible.
An amorphous semiconductor material which has a low energy gap of 0.9 to 1.2 eV with a simultaneously low density of states at a maximum of 5.times.1015 defects/cm.sup.3 eV within the energy gap is, however, unknown. The amorphous semiconductor material, which is suitable for solar cells, with the lowest energy gap to date of 1.4 to 1.5 eV is an amorphous silicon-germanium alloy with a germanium content of up to 30 percent. However, a material with a higher germanium content, and hence a lower energy gap, is necessary for optimum matching to the blue-sensitive amorphous silicon cell. However, such semiconductors, produced using the glow-discharge method, have until now exhibited major material inhomogeneities, a high density of states, and considerable instability with respect to air humidity. The attempt to deposit amorphous germanium layers by reactive sputtering of a germanium target admittedly also produces a material with an improved photo-conductivity, but which has been found to be unstable with respect to relatively lengthy periods of strong illumination (light-ageing).
U.S. Pat. No. 4,471,042 discloses an amorphous semiconductor material for electro-photography, which consists of pure germanium or germanium/silicon alloys.
An article by Lucowski et al. in Physical Review B, Volume 31, No. 4, 1985, pages 2190-2197 discloses thin layers of amorphous germanium and amorphous germanium alloys which are produced in a glow-discharge method in a pure germanium-hydride atmosphere.